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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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FUTEX_WAKE_OP(2const) FUTEX_WAKE_OP(2const)
FUTEX_WAKE_OP - fast user-space locking
Standard C library (libc, -lc)
#include <linux/futex.h> /* Definition of FUTEX_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_futex, uint32_t *uaddr, FUTEX_WAKE_OP, uint32_t val,
uint32_t val2, uint32_t *uaddr2,
uint32_t val3);
This operation was added to support some user-space use cases
where more than one futex must be handled at the same time. The
most notable example is the implementation of
pthread_cond_signal(3), which requires operations on two futexes,
the one used to implement the mutex and the one used in the
implementation of the wait queue associated with the condition
variable. FUTEX_WAKE_OP allows such cases to be implemented
without leading to high rates of contention and context switching.
The FUTEX_WAKE_OP operation is equivalent to executing the
following code atomically and totally ordered with respect to
other futex operations on any of the two supplied futex words:
uint32_t oldval = *(uint32_t *) uaddr2;
*(uint32_t *) uaddr2 = oldval op oparg;
futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
if (oldval cmp cmparg)
futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
In other words, FUTEX_WAKE_OP does the following:
• saves the original value of the futex word at uaddr2 and
performs an operation to modify the value of the futex at
uaddr2; this is an atomic read-modify-write memory access
(i.e., using atomic machine instructions of the respective
architecture)
• wakes up a maximum of val waiters on the futex for the futex
word at uaddr; and
• dependent on the results of a test of the original value of the
futex word at uaddr2, wakes up a maximum of val2 waiters on the
futex for the futex word at uaddr2.
The operation and comparison that are to be performed are encoded
in the bits of the argument val3. Pictorially, the encoding is:
+---+---+-----------+-----------+
|op |cmp| oparg | cmparg |
+---+---+-----------+-----------+
4 4 12 12 <== # of bits
Expressed in code, the encoding is:
#define FUTEX_OP(op, oparg, cmp, cmparg) \
(((op & 0xf) << 28) | \
((cmp & 0xf) << 24) | \
((oparg & 0xfff) << 12) | \
(cmparg & 0xfff))
In the above, op and cmp are each one of the codes listed below.
The oparg and cmparg components are literal numeric values, except
as noted below.
The op component has one of the following values:
FUTEX_OP_SET 0 /* uaddr2 = oparg; */
FUTEX_OP_ADD 1 /* uaddr2 += oparg; */
FUTEX_OP_OR 2 /* uaddr2 |= oparg; */
FUTEX_OP_ANDN 3 /* uaddr2 &= ~oparg; */
FUTEX_OP_XOR 4 /* uaddr2 ^= oparg; */
In addition, bitwise ORing the following value into op causes
(1 << oparg) to be used as the operand:
FUTEX_OP_ARG_SHIFT 8 /* Use (1 << oparg) as operand */
The cmp field is one of the following:
FUTEX_OP_CMP_EQ 0 /* if (oldval == cmparg) wake */
FUTEX_OP_CMP_NE 1 /* if (oldval != cmparg) wake */
FUTEX_OP_CMP_LT 2 /* if (oldval < cmparg) wake */
FUTEX_OP_CMP_LE 3 /* if (oldval <= cmparg) wake */
FUTEX_OP_CMP_GT 4 /* if (oldval > cmparg) wake */
FUTEX_OP_CMP_GE 5 /* if (oldval >= cmparg) wake */
The return value of FUTEX_WAKE_OP is the sum of the number of
waiters woken on the futex uaddr plus the number of waiters woken
on the futex uaddr2.
On error, -1 is returned, and errno is set to indicate the error.
On success, FUTEX_WAKE_OP Returns the total number of waiters that
were woken up. This is the sum of the woken waiters on the two
futexes for the futex words at uaddr and uaddr2.
See futex(2).
EFAULT uaddr2 did not point to a valid user-space address.
EINVAL uaddr2 does not point to a valid object—that is, the
address is not four-byte-aligned.
EINVAL The kernel detected an inconsistency between the user-space
state at uaddr and the kernel state—that is, it detected a
waiter which waits in FUTEX_LOCK_PI(2const) or
FUTEX_LOCK_PI2(2const) on uaddr.
Linux.
Linux 2.6.14.
futex(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-30 FUTEX_WAKE_OP(2const)
Pages that refer to this page: futex(2)
Copyright and license for this manual page
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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